Patent document 1: JP 2010-139927 A
Patent document 2: JP 2009-205102 A (US 2009/0219615 A1)
A head-up display apparatus is known which reflects using an optical reflection face a display image, which is formed on an image formation face of a screen, thereby projecting the display image on a windshield of a vehicle, for instance, as disclosed in Patent document 1. The above screen for the head-up display apparatus has been recently adopted a configuration disclosed in Patent document 2.
With reference to FIG. 9, a screen 130 for projection in Patent document 2 includes a plurality of convex lenses 132 arranged in a grid pattern on an xy-coordinate plane that is also referred to as a reference face. The convex lenses 132 reflect or refract incident lights that fall on the image formation face, emitting the lights with a predetermined diffusion angle. The configuration of the head-up display apparatus in Patent document 1 adopting the screen 130 in Patent document 2 accurately controls the light diffusion of the display image formed on the image formation face so as to reach and cover the whole of the viewpoint region. The display image may be viewed clearly as a virtual display image. The screen 130 in Patent document 2 may be thus suitably adopted in a head-up display apparatus with a high display quality.
The present inventors have found the following disadvantage in the configuration combining the technologies of Patent documents 1, 2. That is, the screen 130 in Patent document 2 has necessarily grooves 139 each having a recessed curved surface in between the adjoining concave lenses 132. The curvature of a convex surface of the convex lens 132 may be small enough whereas the curvature of the recessed curved surface of the groove 139 unfortunately becomes significantly large. Thus, the recessed curved surface of the groove 139 inclines significantly against the reference face, i.e., xy-coordinate plane of the screen 130. This causes a normal line that is normal to a recessed curved surface of the groove 139 to incline significantly against the z axial that is orthogonal to the reference face. Herein, the z axis of the screen may be directed at a position outside of the whole of the reflection face by changing the orientation of the screen. However, a normal line that is normal to the recessed curved surface of the groove 139 may unfortunately hit a position inside of the reflection face.
Such a configuration causes the following situation. For instance, outdoor lights that pass through a windshield may fall upon the reflection face, and be reflected towards the screen. Further, the groove of the screen may irregularly reflect and return a part of the outdoor lights towards the reflection face. Then, the reflection face may project the outdoor lights returned from the screen onto the windshield together with the lights of the display image. As a result, the outdoor lights may be unfortunately reflected on the virtual display image viewed from the viewpoint region.